This invention generally relates to offshore structures for use in severe storm, earthquake and arctic environments and more particularly to structures which are resistant to storm, earthquake and ice loads and can be used for year-round operations.
Exploration and production of hydrocarbon reserves in arctic offshore regions present unique challenges due to the heavy ice cover environment. In these arctic regions, large moving bodies of ice can damage offshore structures such as drilling barges, offshore platforms and underwater pipelines. The ice environment not only presents technical challenges with respect to the design of arctic structures, but can also lead to a very shortxe2x80x94often 3 months or lessxe2x80x94drilling season, and in some areas the early onset of severe storms by mid-October can threaten to further shorten the drilling season.
Encroaching ice can pose a threat to drill ships or existing offshore structures and can cause interruption or abandonment of drilling operations during the short open water period of summer. Ice loads usually govern the design of structures and operations in these arctic environments because they arc likely the most significant loads an offshore structure or operation will face. Thus the ice environment dictates many decisions regarding offshore operations and cost feasibility. As described further below, the oil and gas industry has searched for ways to economically explore for and produce hydrocarbons in this environment. Many of the current methods for overcoming these ice environment problems arc expensive, limited to applications in shallow waters or arc not designed to be used in year-round operations.
One approach of oil exploitation in the ice environment is through the use of artificial islands as drilling and production platforms. Artificial islands are particularly well suited for shallow, near-shore or protected waters. These artificial islands are constructed of sand, gravel or dredged seabed filler material and are designed to resist the ice forces and minimize the erosion effects of summer storms. Drilling rigs and equipment can be brought to the site either by helicopter or trucking over the ice during early winter or by barges during summer. These artificial islands can be cost-competitive with other systems when there is ease of access from land, a suitable filler material is available, and stable ice conditions exist. The volume of filler material required for such an island depends on the work area, type of slope protection required and the water depth. Generally, use of these islands is economically limited to relatively shallow waters or areas with abundant filler material.
For operations in either deeper water s or m ore exposed areas, the volumes of filler material required to construct artificial islands becomes excessive, and thus much more expensive, due to (a) larger ice loads and (b) the natural slopes of the filler material (1:3 for (gravel, 1:12 for sand or silt). As a result, various caisson retained island concepts have been used in deeper waters. Tarsuit and Esso""s Caisson Retained Island are examples of such retained islands. These concepts recognize that, in deeper waters, caissons can substantially reduce island fill requirements. Generally, these concepts use steel or concrete caissons, which provide much steeper slopes than a natural filler material, to form the outer perimeter of the island. The caissons are installed as single or multiple units either on the sea bottom or on a submerged berm, and the island is then formed by filling the core with dredged or other filler material. The caissons therefore reduce the amount of fill volumes required to construct the artificial island and can be used in areas where filler material is not readily available.
For even more exposed offshore sites, ice dynamics and shortened open water periods dictate the use of such novel drilling systems as the Concrete Island Drilling System (CIDS), the Single Steel Drilling, Caisson (SSDC), and the Mobile Arctic Caisson (MAC). The CIDS is a concrete and steel mobile drilling structure which consists of a steel mud base, a central concrete brick positioned in the ice zone, and twin steel deck barges supported on the brick. The SSDC was constructed from a tanker, a segment of which was equipped with a double hull having concrete between the shells, and was ballasted onto a subsea sand berm. The MAC is a caisson which consists of a continuous steel ring on which sits a selfxe2x80x94contained deck structure. The core is filled with sand to provide horizontal resistance, and the MAC is designed to sit on a submerged berm in depths over 70 feet, but can operate without a berm in depths ranging from 30-70 feet. These systems are generally large monolithic systems which are constructed and fully outfitted with drilling equipment in a temperate environment and then towed to the desired arctic location.
These systems (SSDC, CIDS, and MAC) have been successfully deployed for exploratory well drilling during the relatively short drilling, season in the Canadian and Alaskan Beaufort Sea. However. even these newer systems are still limited in their capabilities of addressing both greater water depths and extreme ice and wave loads. Because of their large size, these systems are subject to comparably large ice and wave loads, resulting in increased design and construction cost to address those loads. All three would have to be installed on man-made berms designed for the selected system""s foundation in order to withstand year-round operations. These systems are limited by water depth because the construction of the subsea berms becomes very costly and time consuming as water depth increases. Also, the freeboard of the three systems actually built were specifically designed for the protected regions of the Beaufort Sea, and would be subjected to wave over-topping in unprotected environments. Extensive protective measures, such as wave deflectors, would have to be installed to protect certain regions of the deck from wave slamming and wave overtopping. As a consequence of these limitations, development of hydrocarbon reserves in certain arctic regions may be uneconomic using, these systems.
Other mechanisms for handling, ice loads have been proposed including various barrier-type mechanisms which are used to protect existing offshore structures. For example, U.S. Pat. No. 4,523,879 (Finucane et al.) discloses a method for constructing spray ice barriers to protect offshore structures in a frigid body of water from mobile ice, waves and currents. U.S. Pat. No. 4,504,172 (Clinton et al.) discloses a caisson shield consisting of an essentially annular concrete structure which encircles at least the submerged support section of an offshore production platform. U.S. Pat. No. 5,292,207 (Scott) discloses a submersible mobile gravity based caisson which can be used to protect existing semi-submersible mobile offshore drilling units and mobile offshore oil well production rigs which are ice crush sensitive. These protective devices are generally limited to use in shallow and/or calm waters. One limitation with spray ice barriers is that they are not feasible when the ice is very dynamic. Also, the use of such barriers is limited to relatively shallow waters to ensure that the spray ice will be firmly groundedxe2x80x94which is necessary to provide protection. The other protective barriers are extremely costly where the wave environment is severe: they attract significant wave loading, and the cost for marine operations to deliver and set the barriers is relatively high.
Various ice resistant offshore platform structures have also been proposed for operating in the harsh arctic environment. For example, U.S. Pat. No. 4,048,943 (Gerwvick, Jr.) discloses a floating caisson that can be actively heaved in the water to break-up encroaching ice. U.S. Pat. No. 3,793,840 (Mott et al.) discloses a mobile arctic drilling and production platform having a controllably buoyant foundation-like base to afford a firm footing at its lower end which normally rests on the ocean floor. A conical shell-like body extends upwardly from the base to provide a widespread footing for the platform in conjunction with the base. A caisson extends through the platform and is partially embedded in the substratum beneath the platform to assist the platform in absorbing and transmitting to the ocean bottom the lateral forces imposed on the structure and to protect wells during and after drilling operations. Conical structures, such as the two reference above can be useful in a severe and dynamic ice environment and can be designed for a wide range of water depths. However, they tend to become very expensive, and because of the large conical shape, it is often difficult to install the deck and have access to the stricture for resupply.
For the foregoing reasons, persons skilled in the offshore petroleum industry will readily understand the economic incentives for a low-cost drilling and production platform system that is capable of year-round operations in severe storm, earthquake and ice environments. It would be further advantageous if such systems could be of relatively small dimensions to minimize ice loads and material quantities, easy to construct, and quickly installed and abandoned in response to changing ice conditions. As described further below, the present invention provides a system capable of meeting these needs.
The foregoing disadvantages of the previously proposed techniques and structures are substantially eliminated through the various embodiments of the present invention. In one embodiment, the present invention generally comprises an offshore structure for use in an offshore arctic environment in which moving ice sheets and other dynamic masses of ice are present. The offshore structure includes a tubular caisson structure having a lower foundation section and an upper section which are separated by a structural diaphragm. The lower foundation section extends downwardly from the seafloor into the seabed a distance to provide sufficient lateral and vertical soil resistance to resist lateral and vertical loads on the offshore structure. The upper section extends upwardly from the seafloor to a point above the surface of the body of water and is adapted to support a deck structure on the upper end. The structural diaphragm is adapted to rest on the seafloor when the offshore structure has been fully installed to enhance the lateral and vertical load carrying capacity of the tubular caisson structure. In a preferred embodiment, there is a means for creating suction in the lower foundation section during installation to assist the caisson in penetrating into the seabed. When used in ice environments, the offshore structure may include an optional ice resistor.